The invention relates to a steam turbine and more particularly to platform motion restraints for freestanding turbine blades when the turbine is on turning gear.
When the turbine is operating, the blade roots are held tight in the rotor blade grooves by centrifugal force, but when the turbine is on turning gear rotating at about 3 revolutions per minute there is negligible centrifugal force. Gravity pulls the distal ends of the blades downwardly when in the 3 and 9 o'clock positions producing a rocking motion at the root due to the required clearances between the blade root and the rotor grooves. This repetitive small motion between the blade root and rotor groove causes fretting, a combination of wear and corrosion, that leads to cracking in the highly stressed root area of the blade.